1. Field of the Invention
The present invention relates to an image processing device for performing signal processing based on human visual characteristics in an imaging device such as a digital camera, the signal processing being compression of a dynamic range, gray level correction, contrast enhancement, or the like.
2. Description of the Prior Art
When an image of a person is captured under a backlight condition by an imaging device such as a digital camera, image data obtained by capturing the image includes luminance information of a wide dynamic range from a dark portion to a bright portion. Therefore, some of imaging devices such as digital cameras include an image processing device for correcting such image data to have appropriate image quality.
As an example of such an image processing device, known is an image processing device adopting a visual processing technique, wherein according to luminance distribution of surrounding pixels around a pixel under correction processing (target pixel), the luminance of the target pixel is corrected to be increased in a dark portion and to be lowered in a bright portion (for example, see Japanese Published Unexamined Patent Application No. 2006-24176, which is hereinafter referred to as Patent Document 1).
In the image processing device of Patent Document 1, first, an unsharp signal US is obtained by a low-pass spatial filter which passes only a low frequency component of an input signal. The unsharp signal US shows luminance distribution information of the surrounding pixels around the target pixel (specifically, a center pixel of the low-pass spatial filter). Moreover, the image processing device includes a two-dimensional LUT (look up table), in which matrix data referred to as profile data is registered. The profile data includes rows respectively corresponding to various pixel values of the input signal and columns respectively corresponding to various pixel values of the unsharp signal US. Pixel values of a visually processed signal Si according to the combination of the input signal and the unsharp signal US are stored as factors in the rows and columns. Then, the image processing device outputs a corrected pixel value with reference to the two-dimensional LUT. In this way, according to the luminance distribution of the surrounding pixels, an appropriate brightness adjustment can be performed on a pixel-by-pixel basis.
Moreover, as another example of the image processing device, known is an image processing device in which image data is divided into a plurality of block areas for each of which a gain value is calculated according to a histogram as to luminance information of the image data and histogram equalization processing is performed to accordingly change a luminance value by the gain value, and noise reduction processing is performed on each of the block areas with a strength according to the gain value (for example, see Japanese Published Unexamined Patent Application No. 2006-50042, which is hereinafter referred to as Patent Document 2).
However, in the image processing device of Patent Document 1, when image data whose luminance value has a wide dynamic range is subjected to visual processing, a gain of a noise component in a pixel whose gain is increased through the visual processing is also increased. As a result, in the corrected image data, the noise component can be remarkable. That is, performing the visual processing may deteriorate the image quality by contraries.
For example, visual processing is performed on image data whose luminance value has a wide dynamic range as in a backlight condition in which a person is standing against a background of a window with a bright landscape. In this case, a gain of only an area corresponding to the person is increased so that the area corresponding to the person can become bright. As a result, the area corresponding to the person becomes bright, but a gain of a noise component in the area corresponding to the person is also increased.
On the other hand, since the image processing device described in Patent Document 2 performs the histogram equalization (luminance change) processing and the noise reduction processing according to the gain calculated based on the histogram, the noise reduction processing is performed with a strength according to a strength of the histogram equalization.
However, the processing mentioned above is performed on a block-by-block basis. Therefore, if luminance values of pixels in a block have a wide dynamic range, the effect of histogram equalization may be too strong or insufficient in some of the pixels. Moreover, if the noise reduction processing is performed with a uniform strength on all the pixels in a block, noise in pixels having a number of noise components may not be sufficiently suppressed, or by contraries, an edge of an area having a small amplitude of noise may become dull. To solve the problem, it can be conceived to reduce the block size as much as possible. However, as the block size decreases, the amount of data which has to be processed to determine the histogram increases. That is, a reduction of the block size has a limit, and thus it is difficult to realize both appropriate visual processing and suppression of noise (prevention of image deterioration).